The Linear Tape Open (LTO) format is an open format magnetic tape data storage technology that enables users to have access to multiple sources of storage media products that will be compatible with one another. In current LTO tape drives, variable-length blocks of user data are received from a host interface and are segmented to create fixed-size data blocks or data sets. These data sets are further broken down into smaller but equal-size units known as subdata sets (SDSs). An SDS is typically organized into a two-dimensional array of rows and columns of data symbols. Each row of data symbols in the two-dimensional array may be made up of multiple interleaved data and parity symbols. Error Correction Coding (ECC) is typically generated for each row and each column in the array to protect the data contained therein. More specifically, ECC parity bytes are generated for each row that are appended to each row to create multiple codewords (e.g., four for LTO 6 and LTO 7 drives and likely to increase in future generations) that include both data and parity symbols. Each row of encoded SDS is referred to as one codeword interleave (CWI). Additionally, CWIs are also encoded with ECC parity bytes for each column that are appended to each column to generate vertical protection for the segmented user data, summing up to N CWIs per SDS.
The rows of each SDS, i.e., the CWIs, with the possibility of header and other metadata information having been added thereto, are distributed across and along the tape in a number of passes called wraps. In each wrap, based on the number of tracks T, CWIs are allocated to each track such that spatially correlated errors or defects on the magnetic tape surface will spread across multiple SDSs. Stated in another manner, in order to ensure that the number of errors in an SDS do not overpower the ECC codes used to protect the SDS, the rows of the SDS may be laid out on the magnetic tape in such a manner that, if errors occur spatially close to one another on the tape medium, the errors will be spread across multiple SDSs in the data set. Thus, in an attempt to minimize the burden on the ECC decoding of each SDS, such an allocation will ideally even out the correlated errors that happen on magnetic tape by distributing CWIs over the distinct SDSs. Limiting the number of errors occurring in an SDS increases the probability that the ECC parity associated with the SDS will be powerful enough to correct the errors contained therein.
Recent developments in multi-track LTO tape drive systems paved the way for storing petabytes of data at very low costs as part of the green storage context in large scale deployments. This performance gain is due to various innovations that took place in further research for media characteristics, tape mechanics, head technology, tribology and advanced signal processing algorithms. Most of the today's tape technology relies on continuous operation in different conditions and environments in which the media and the data are subject to defective reads/writes and external wear and damage. Most of the survey data and experimental observations demonstrate that the majority of performance bottleneck is due to such external repeatable defects and associated correlated failures such as dead tracks. Physical constraints of tape and the guarantee of operation under different environmental conditions have led to advanced configurations such that the performances of signal processing and error correction coding algorithms are affected the least. One of the genuine features of the LTO format is in the way coded data is laid out along and across the magnetic tape surface.
In one application of such a tape layout process, LTO tape drives employ a set of “randomization” methods (also sometimes referred to herein as “layout parameters”) in order to balance the distribution of CWIs on the tape surface. Such methods include, but are not limited to, track rotations (transverse to the tape), CWI set swaps, track swaps and odd/even indexed SDS separations, which are designed to evenly distribute CWIs on the tape and thereby decorrelate error locations on the tape from error locations within each SDS. CWIs from an SDS may be periodically swapped between even and odd data tracks because even data tracks and odd data tracks may have systematic differences. Such systematic differences may be the result of recording head design, electronics configuration, signal line routing, or the like.
One drawback that has been seen with such methods is that they do not substantially improve the separation distance between CWIs to achieve optimal decorrelation. Subsequent efforts have proposed to maximize the minimum separation distance between CWIs belonging to the same SDS, while evening out the CWI set and track distribution, in order to make each SDS have almost the same decoding performance and data reconstruction reliability. Unfortunately, such subsequent efforts have also experienced certain drawbacks, as optimal decorrelation of errors is not always achieved depending upon the types of defects being seen.